Types of RNA

There are different types of RNA, each of which plays a specific role, including specifying the amino acid sequence of proteins (performed by messenger RNAs, or mRNAs), organizing and catalyzing the synthesis of proteins (ribosomal RNAs or rRNAs), translating codons in the mRNA into amino acids (transfer RNAs or tRNAs) and directing many of the RNA processing steps (performed by small RNAs in the nucleus, called snRNAs and snoRNAs).

All of these types of RNAs begin as primary transcripts copied from DNA by one of the RNA polymerases. One of the features that separates eukaryotes and prokaryotes is that eukaryotes isolate their DNA inside a nucleus while protein synthesis takes place in the cytoplasm. This separates the processes of transcription and translation in space and time. Prokary-otes, which lack a nucleus, can translate an mRNA as soon as it is transcribed by RNA polymerase. As a consequence, there is very little processing of prokaryotic mRNAs. By contrast, in eukaryotic cells many processing steps occur between mRNA transcription and translation. Unlike the case of mRNAs, both eukaryotes and prokaryotes process their rRNAs and tRNAs in broadly similar ways.